Increasing evidences show the importance of astrocytes in the development of neurodegenerative pathologies. In Huntington's disease (HD), neurodegeneration is strongly influenced by the toxicity linked to the production of mutant huntingtin (mHtt) in both neurons and astrocytes. The latter have numerous roles in supporting neurons such as maintaining the blood-brain barrier, regulating the blood flow, the homeostasis of ions and water, the synaptic transmission and the energy metabolism. Nevertheless, the concrete role of astrocytes into neuronal dysfunction and death in HD pathology remains to be determined.
To overexpress a gene specifically in astrocytes, most strategies rely on constitutive or conditional transgene expression in astroglial cells. Among these, one may cite tamoxifen (TAM) inducible CreERT2/loxP system or a constitutive expression of the transgene from an astrocytic promoter such as the GFAP-HD mice model. In these models, the transgene is expressed in astrocytes of almost all brain regions. However, in GFAP-CreERT2-transgenic mice, recombination in neurons is also occurring, since the GFAP promoter is active in embryonic radial glia that possesses a substantial neurogenic potential. In addition, GFAP-driven transgene expression in striatum, the main affected brain area in HD, is very low under physiological conditions. Differences in recombination efficiency between transgenic lines complicate the analysis and it is still not always possible to target specific cell subpopulations through transgenesis. Hence, studying the specific role of astrocytes in HD requires the development of tools specifically targeting this particular cell population in one targeted brain structure.
The development of highly efficient viral vectors for gene transfer in the CNS is providing new systems for localized and controlled gene expression in a subset of cell population (Jakobsson and Lundberg, 2006; Wong et al., 2006). Pseudotyping lentiviral vectors (LVs) with the glycoprotein of the vesicular stomatitis virus (VSV-G) confer a high neurotropism when combined with the ubiquitous promoter of the phosphoglycerate kinase 1 (PGK) (Deglon et al., 2000). In contrast, LVs pseudotyped with the glycoprotein of the Mokola virus (MOK-G) transduce both neurons and glial cells (Cannon et al., 2010; Desmaris et al., 2001; Pertusa et al., 2008; Watson et al., 2002). Furthermore, it has been shown that addition of a micro-RNA target (miRT) restricts transgene expression to cell subpopulations (Brown et al., 2007a; Brown et al., 2007b; Brown et al., 2006). Combining MOK-G pseudotyping with miR124T detargeting is associated with an astrocytic targeting of LVs (Colin et al., 2009). Colin and colleagues have used miR124, which is highly and specifically expressed in neurons (Deo et al., 2006; Lagos-Quintana et al., 2002; Mishima et al., 2007; Smirnova et al., 2005) to overexpress genes of interest into striatal astrocytes in vivo. However, this system is not suitable for silencing a ubiquitous gene because of the maturation of a small interfering RNA (siRNA) by dicer which cleaves the miRT located at the 3′-end of the mRNA and prevents detargeting, as is shown in FIGS. 10A and 10B.